The present invention relates generally to optical encoders, and more specifically to an improved optical encoder incorporating a reliable simplified construction, low overall component count, and a component arrangement which reduces errors potentially arising in conventional devices from relative component displacement and light source degradation encountered during operation. An optical encoder constructed according to the present invention achieves increased resolution as one result of these combined features.
Optical encoders are known for use in industrial motion and position sensing applications, such as motor control devices, fluid flow control devices, motion sensing and control devices operating through a shaft, and panel mounted test and/or measurement devices. Consumer applications include such things as computer printers, disk drives, card readers, and ticket dispensers. Generally, optical encoders are potentially useful in any device requiring a speed sensing function, where the speed to be sensed can be linked to a shaft of the optical encoder. In a fluid flow sensing application, for example, the shaft of the optical encoder may be linked to a turbine disposed in the flow of fluid to provide flow rate or volume measurements. The encoder may provide flow direction information as well.
Such encoders typically include a housing enclosing an axially rotating shaft. An opaque code wheel having light transmissive slots is secured to the shaft between a light source and a light detector. Typically, at least two photodetectors are disposed within the housing on the opposite side of the code wheel from one or more light emitting diodes (LEDs). The path of light from the LEDs to the photodetectors is interrupted by opaque portions of the code wheel as the code wheel rotates with the shaft to present a modulated light signal at the photodetectors.
The waveform sensed by the photodetectors therefore reflects the frequency with which the slots in the code wheel permit light to reach those detectors, and is indicative of the speed of shaft rotation. Use of two phase offset sensing channels also permits determination of the direction of shaft rotation.
The present invention relates -to relatively low cost encoders, selling in the range of $20 each, for which the manufacturing tolerances are such that the shaft is subject to radial displacement within its support bushing. This displacement tends to skew the code wheel's orientation relative to the detectors. Accordingly, the transmissive slots in the wheel have a range of misalignment relative to the photodetectors.
Conventional optical encoders typically mount the photodetectors individually in the housing, which increases the potential for misalignment, which in turn reduces the amount of light transmitted to the photodetectors. Because the photodetectors integrate the light signal impinging upon the detectors to produce an electrical signal, a reduction in the amount of light through misalignment increases the minimum response time. The potential for component misalignment is therefore a primary limitation on the resolution of a particular encoder.
In the case of conventional encoders, when the designer or operator needs to change the resolution parameters, the relative positions of the two photodetectors must be changed. This is an operation which is difficult to perform accurately. As such, the resolution of conventional encoders is not readily adjusted.
Yet another disadvantage of conventional optical encoders is that they are susceptible to contamination from dirt and debris which infiltrates the housing and interferes with the modulation and detection of the light beams. As a result, the operational performance of the encoder is reduced because the light intensity of the LEDs is diminished. An additional hazard caused by the infiltration of dust is that the rotational movement of the encoder wheel shaft may be impaired.
Still another disadvantage of conventional encoders is that as many as 35 separate components are required to complete the assembly. Many of these components are required for the positioning and securing of the principal components mentioned above within the housing.
A further disadvantage of conventional optical encoders is that upon the mounting of a cable to the output connector pins or leads, the cable is subject to mechanical strain due to physical manipulation, which has the potential to damage or bend the output leads.
Accordingly, it is an object of the present invention to provide an improved optical encoder having a mechanically improved assembly of fewer components which reduces misalignment difficulties compared to conventional encoders, thereby permitting increased resolution.
A further object of the present invention is to provide an improved optical encoder which uses at least two light detectors mounted upon a single chip, and in which the chip is secured to a housing using locating studs formed integrally with the housing, thereby reducing the potential for relative displacement between the light detectors themselves and between the light detectors and other components of the optical encoder.
A still further object of the present invention is to provide an improved optical encoder having a sealed housing to reduce the potential for infiltration of performance degrading dust and other contaminants within the encoder.
Another object of the present invention is to provide an improved optical encoder in which resolution may be adjusted merely by changing the slot pattern of the encoder wheel.